srs_enb: Make num_prb a config parameter

Change-Id: I336e71f53556596050cf3eb55a7810a009dbf7a8
diff --git a/example/defaults.conf b/example/defaults.conf
index f32e1f5..0bff86a 100644
--- a/example/defaults.conf
+++ b/example/defaults.conf
@@ -100,3 +100,4 @@
 srsenb:
   mcc: 901
   mnc: 70
+  num_prb: 100